The bars on the graph show average precipitation differences by decade for relative to the average. The far right bar is for However, natural factors cannot explain the recent observed warming. All of these natural factors, and their interactions with each other, have altered global average temperature over periods ranging from months to thousands of years.
The difference is that, since the beginning of the Industrial Revolution, humans have been increasingly affecting global climate, to the point where we are now the primary cause of recent and projected future change. The majority of the warming at the global scale over the past 50 years can only be explained by the effects of human influences, especially the emissions from burning fossil fuels coal, oil, and natural gas and from deforestation.
Carbon emissions from burning coal, oil, and gas and producing cement, in units of million metric tons of carbon. Data from Boden et al. The emissions from human influences affecting climate include heat-trapping gases such as carbon dioxide CO 2 , methane, and nitrous oxide, and particles such as black carbon soot , which has a warming influence, and sulfates, which have an overall cooling influence.
In addition to human-induced global climate change, local climate can also be affected by other human factors such as crop irrigation and natural variability. Oil used for transportation and coal used for electricity generation are the largest contributors to the rise in carbon dioxide that is the primary driver of recent climate change. Carbon dioxide has been building up in the atmosphere since the beginning of the industrial era in the mids, primarily due to burning coal, oil, and gas, and secondarily due to clearing of forests.
Methane levels in the atmosphere have increased due to human activities including agriculture with livestock producing methane in their digestive tracts and rice farming producing it via bacteria that live in the flooded fields ; mining coal, extraction and transport of natural gas, and other fossil fuel-related activities; and waste disposal including sewage and decomposing garbage in landfills.
Other heat-trapping gases produced by human activities include nitrous oxide, halocarbons, and ozone. Nitrous oxide levels are increasing, primarily as a result of fertilizer use and fossil fuel burning. The conclusion that human influences are the primary driver of recent climate change is based on multiple lines of independent evidence. The first line of evidence is our fundamental understanding of how certain gases trap heat, how the climate system responds to increases in these gases, and how other human and natural factors influence climate.
The second line of evidence is from reconstructions of past climates using evidence such as tree rings, ice cores, and corals. These show that global surface temperatures over the last several decades are clearly unusual, with the last decade warmer than any time in at least the last 1, years and perhaps much longer.
The third line of evidence comes from using climate models to simulate the climate of the past century, separating the human and natural factors that influence climate. When the human factors are removed, these models show that solar and volcanic activity would have tended to slightly cool the earth, and other natural variations are too small to explain the amount of warming.
Only when the human influences are included do the models reproduce the warming observed over the past 50 years. In addition to such temperature analyses, scientific attribution of observed changes to human influence extends to many other aspects of climate, such as changing patterns in precipitation, increasing humidity, changes in pressure, and increasing ocean heat content. Boden, T.
Marland, and B. Forster, P. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D. Fahey, J. Haywood, J. Lean, D. Lowe, G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz, and R. Van Dorland , Ch. Solomon, Qin, D. Francis, J. Vavrus , Evidence linking Arctic amplification to extreme weather in mid-latitudes.
Geophysical Research Letters , 39 , L, doi Frisvold, G. Jackson, J. Pritchett, and J. Ritten , Ch. This is used for graphs that illustrate projected future changes simulated by climate models. The purpose of these graphs is to show projected changes compared to a period that people have recently experienced and can remember; thus, the most recent available year period was chosen the historical period simulated by the CMIP3 models ends in or Karl, T.
Melillo, and T. Karl, Melillo, J. Cambridge University Press, pp. Meehl, G. Washington, T. Wigley, J. Arblaster, and A. Dai , Solar and greenhouse gas forcing and climate response in the twentieth century. Journal of Climate , 16 , , doi Some figures and images are copyright protected. Permission of the copyright owner must be obtained before making use of copyrighted material. Skip to main content. Top GlobalChange. The assessment criteria are proposed as follows:.
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